Can I use more sand topping mortar to level the slope of a shower pan?
is adding mortar going to work?


Yes, if you do it correctly. Rough up the existing surface where you intend to build up the pan height, then moisten well prior to adding some dry-pack to level off the area. I would probably use a "wetter" dry-pack mix for this (as compared to the "drier" dry-pack mix I used to originally create the bed). We often do some "tune-up" to the bed by shaving, scraping, moistening and patching, etc. even days later.

I would not use thinset mortar to build up the bed due to the thickness involved (3/4"). Not to say it wouldn't work though; and no I would not use that to set tile, you would trowel it smooth and level then let it cure. Then set tile as per normal with notched trowel.