The entire tailgate 'shell' and internal structure is different with / without the step. It's not possible to add the pullout step to a tailgate that didn't have it as OE.If you can get the cap off the top and see if there is space inside it may be possible.
If referring to an in-bed camper with a rear door, IMO the tailgate step would be the more useful of the two types.I’m getting a camper and the step would be super useful.
I think you'll find there areHowever some are powered up and down gates and I believe there are also manual gates with the step as well.
I have the AMP Research BedStep and love it. I use it quite often with the tailgate up. My neighbor has an F150 Lariat with the step built into the tailgate and comments that he wish he didn't have it. It makes the tailgate heavier and is tired of the effort it takes to close. He is amazed how light and easy my tailgate is to operate.I thought I had to have a tailgate step but glad I went with a AMP Research BedStep. Easy to extend and retract with foot. Also can hold onto bed side when entering and exiting and can use with trailer attached.