I'm trying to add an additional feature for one of my players for their +1 mace that also does an additional 1d4 against any undead creature. Is there a way to add that or am I stuck writing it down in the descriptions and letting my player just add that during the proper encounter/roll?

This is how all of the official Magic Items that are similar were programmed. Add the following Modifier to the Mace, +1:

• Modifier—>Damage
• Modifier Subtype—>[select damage type]
• Die Count—>1
• Die Type—>d4

• Details/Restictions—> “against undead”

After you save the modifier, re-save the whole magic item. Then just be patient for the changes to push through to the character sheet.

Thanks!

Happy to help