Anything is possible with powerstore since you have access to all of the code... however some functionality isn't built in and requires a code update.
Adding a % markup field to the entire store that is editable would be pretty easy... however it would require manual update of code in several locations, so I'd recommend using a premium support ticket to get it done if you aren't perfectly familiar with writing php code.
Doing it more granular on a product by product basis, by category, or based on the starting price would be slightly more difficult, but we could also help with this in a premier ticket... just make sure you have a clear idea of the ranges and options you want and sign up and we will set up a time to help you through it.
Did this help? Tips are appreciated...
