Possible, yes. But HVAC systems are balanced to some extent; adding registers willy nilly could affect airflow in the rest of the house. Also, a screened porch (I'm assuming) is 3-sides open, which means you need a decent amount of registers to be effective. Further, you might need an airflow return if the doors can close.
The point is, don't assume it's as simple as adding a register or two. Have an HVAC man come and estimate what needs to be done.