How exactly would you like to show the tax inclusive price? Would you like it show this instead of the regular price on the details page, or in the cart?
Do you want tax to show as a subtotal for all of the items? Please describe as best as you can how you would like this to look from the perspective of the user that will be adding the items.
In general you can edit about any part of the PowerStore if you have the rite tools and info, but depending on exactly how you would like this to work it may require some custom coding.