The 10mm of something will either be a sand/cement render or some form of a base coat plaster. You can’t just use skim plaster (Multifinish) that thick as it will craze & fall off. Assuming you’ve bricked up the doorway leaving the blocks approx. 10-12mm mm. below the surrounding plaster; you need to chip away the edge of the plaster all around the old door opening where it meets the blocks to give a rough edge, this helps to stop cracking where the new plaster meets the old.
You then have a couple of choices:
Build up the door recess using Hardwall plaster but you need to do it in 2 layers or it will crack; if you’re going to tile over the plaster you can plaster out to the same level using Hardwall as this will give a finish good enough for tiling onto. Don’t try & get it smooth immediately, just get it up there & then drag a straight piece of wood (or metal if you have it) across the door opening top get it level with the surrounding area, then you can trowel it flat & smooth but that’s where the tricky bit comes in!
Alternatively, you can leave the second layer of Hardwall 2-3mm below the surrounding wall & skim it level using Multifinish plaster.
