You cannot yet enter Unicode values yet but you can paste the actual Unicode character - and view it Unicode value too: Adding Special Characters & Adding Unicode - #2 by franz
As for replacing a glyph, just copy the original Unicode character (it’s under each glyph - and it might look like a square if it’s a PUA) then select the new shape (eventually along with a horizontal guide to precisely define its baseline), paste the character in the field “Type any character”, then press Create Glyph and Replace:
Hope this helps.