Can I buy an aluminum seatpost with a chromoly frame
There isn't a problem with putting an aluminum seatpost in a steel frame. It's a pretty common thing to do.

Make sure to grease the seatpost before putting it in, lest it get stuck. If it does get stuck, you could try ammonia or drano (sodium hydroxide) or chilling the thing (e.g. with a CO2 cartridge to unstick it). See this page for other stuck seatpost suggestions.  

Also, make sure you have the right diameter seatpost. This can be done by checking the documentation for the frame, or using a seatpost measuring tool (which most bike shops should have), or a pair of vernier calipers.