dude at first i thought it was ghost face but when i saw it was slendy.......im not gonna lie i nearly shit myself........damn he scares me.................damn slendy.....poppin up everywhere and shit.......scaring people.......man.......the fuck is wrong with you?
All it'd really need is a mask - probably an accessory, like glasses - that covers the whole face giving a blank effect (wearable in all categories), a matching skintone, and a suit. There's probably a usable suit in the game already.
I do know S2 had a cheat that allowed one to stretch and shrink sims to vary their height a little - there was a specific range recommended that would keep them in the range of "normal". A really slender Slenderman would be doable with that, but I have not seen an equivalent for S3.
A skintone and suit are easy, and the mask shouldn't be really hard to do. Except that I don't know how to mesh, nor how to make accessories. But the way you say it makes me think that it's definitely possible to make a slender man. After all, there are Pyramid Head accessories available