Trying to grow a deciduous, woody shrub as an indoor plant is not likely to be successful. The clerk at Lowe's is just trying to get rid of a plant that they should not be carrying.

It would not be impossible to protect it in a cold storage environment, however.

I am the poster who started that crape myrtle/container thread you saw. Although that discussion was about transplanting a crape myrtle from the ground to a container, it was also about transplanting one of the ones already in the ground to another in-ground location. We have four crape myrtles in the ground here, and even though we live in Central California, we had freezing temps in the 20s last winter. I don't know how cold it gets where you are, but all of our crape myrtles made it through that month-long freeze just fine. Just thought I'd mention it in case you were thinking of doing it in-ground.

It gets bitterly cold some days in Iowa. -20ish at the coldest. I had planted several zone 5 crape myrtles last year and a few zone 6 that the garden center (reputable not Lowe's) said would be able to be returned if they didn't come back. Not one of them came back. I adore CM's. I fell in love with them on a trip to Georgia two years ago.

I have grown a crape myrtle (the dwarf "Victor") for almost 10 years and hauled it in and out each year. Unfortunately due to the PM that would ravage that type every year (seemingly resistant to either the most basic baking soda solutions or actual PM drenches or sprays), it eventually had had enough (and I had had enough of it! lol) so I finally tossed it this past winter.

I kept it in my darkish, cool (avg temp ~60° F - 65° F) bedroom each winter, it would drop its leaves and stay dormant, and then around February, it would start putting out new growth with the increasing daylength. The tricky part would be to try to ease it out in spring/summer and let the inside foliage adapt to the outside, but that is something that must be done with any plant that will go out in summer.

Just like many many people overwinter plumerias, bananas, palms, tropical hibiscus, etc., there is nothing stopping you from doing the same with a crape myrtle. In the case of the crape, as a warm temperate shrub, if you have a cold, non-freezing spot (maybe an attached garage, shed, crawl space, or root cellar that stays cold but remains around or above freezing and up to the 40s or low 50s), then you could keep a potted crape in there during winter and bring it out in spring to resprout.

Note that crapes bloom on new growth so there is nothing stopping you from doing major top pruning on it in late winter/early spring to keep the height in check. In many colder areas in the NE where they are inground or in more southerly areas with a freak frost or freeze, they may die back to the roots and resprout as a bush, but they bloom just fine just the same.

In general, I will overwinter and grow tropicals/subtropicals indoors "as houseplants" (like my tropical hibiscus, DC banana, and coffee arabica) that will go out in summer, but for my marginal-when-potted temperates like the crape, and some others like my baby american beautyberry (or any baby shrub), my gloriosa lily, and some of my Salvias, I have induced dormancy in them and overwintered them in a cool dark spot, keeping them barely moist.

BVE,
Living in zone 5b I can tell you that most crape myrtles do just fine here in central Missouri as long as you get them established well before winter sets in. Mulch them heavily after the first good frost and make sure the soil is moist before winter sets in. When they start to show signs of life in late spring, that's when you can prune them to get rid of any dead top growth.

The main thing to remember about crape myrtles in a zone 5-6 climate is that they will never be the "trees" that you see in warmer climates. After warmer winters in zone 5 they may have more top-growth that survives, so in those years they'll at least take on large shrub proportions. But after more typical 'cold' winters, you'll have to treat them like perennials that are trimmed back nearly to the ground (or mulch line). In those years they'll look more like 3-4 foot flowering mounds. Crape myrtles certainly have their uses in colder climates, you just have to know how to deal with them and where to appropriately place them in the landscape. Just remember to pull away that excess mulch in spring after the frost danger is gone so you don't rot out the trunk.

Regards,
Dan