What can I do having power of attorney for my stepdaughter while her father is overseas?

If there is no court order establishing parental responsibilities between the parents, it is unlikely that your husband has an authority to give you a power of attorney that is any good for anything. But, even if the power of attorney is valid for most matters involving third parties, it doesn't have any validity against the other parent unless there is a court order giving your husband exclusive parental authority (what many states other than Colorado call "sole legal custody"). It isn't clear what you mean by saying the mother "wants to put him on child support", but until there is a court order stating who is responsible for what financial obligations there is no clear obligation. That should have been done long ago, so there is no legitimate complaint about the mother finally getting around to doing something.

She has a right to get support for her child. However, your husband (through you) can request a stay of any orders as he's active duty military. However, he would then be in arrears for child support back to the date of filing when he gets back.

I need many more facts to give informed advice.