It depends, sometimes yes, sometimes no. It depends on the unit. Many, such as heatilator brand offer the same unit as either gas or wood. Many units are also offered as gas only and cannot be made to burn wood. You need to contact the factory authorized dealer/installer for your particular unit. Usually this can be found in the yellow pages or online without too much effort. If you are in luck, then you have an either/or unit that might be converted. You would have to have them remove the gas lines and burner and reseal the gas knockout on the side of the unit. The biggest drawback is that woodburning units need to have a chimney that goes all the way up through the roof. If you don't have this now, then you MUST have one installed that is correct for your unit. Also, you need a hearth. This is a permanently installed, non flammable surface that covers the floor. These are usually 16-20" wide and extend to at least 12" past the opening on either side. This is to catch any wayward hot embers.
What you propose is pretty ambitious and the chances are that it can't be done. You have just taken your first step into the wonderful world of wood and chimneys etc. There is a lot to learn and if you don't do it right then your house can go poof! - Up in smoke... Good luck and be careful.