Yes, you could file returns for other businesses just like any other paid tax preparer, and you are allowed to file them as a Non-Paid Preparer

Youd have to check with those states, I know NY requires you to have a preparer registration number if you're not an EA/CPA, I feel like theres one other one too, but I cant remember which.   

OR and CA have some preparer requirements but only if YOU are located IN those states or if youre soliciting people in those states for tax preparation.