I am trying to create a feat that replicates something similar to Fey Touched, Shadow Touched and Magic Initiate. 

By that I mean I would like to give one free casting of a spell as well as add it to their spell list of always prepared spells so that it may be cast with spell slots as well. below is the written effect I am trying to replicate

You always have these spells prepared, and you can cast each of them once at their lowest level without expending a spell slot. When you do so, you can’t cast them in this way again until you finish a Long Rest. To cast any of these spells at a higher level or a second time prior to finishing a Long Rest, you must expend a spell slot.

I can add the single casting of the spell no problem but am having trouble figuring out how to have the feat add the spell to the prepared spells, if anyone can point me in the right direction I would be thankful