Umm so if you just want HRE to be able to build them, I'm not quite sure why you are working with textures.

Anyway, go the EDU and add HRE to the list of factions to which it is available if its not currently listed.

Then in the EDB, copy/paste a line for French Pikemen (Pikemen) into the building where you want to be able to hire the unit of your choice. Then rename the reference name Pikemen into the name of your unit. Now it is hirable at this building in your campaign.