Hi,
Yes, anything is possible if you know how. It’s quite common to cut out the sill where rotten, and splice in a new piece shaped to match the surrounding sill. Alternatively, the complete front section (protruding part) of the sill can be removed, and replaced, assuming that the remaining part of the wood from the face of the window is sound enough for the new sill to screw into. Don’t forget that the new sill needs to be sloped, with a weather drip (groove) to the underside.
At one time it was quite common for us to replace the complete sill, but it does mean removing or smashing the glass, and saw cut the both jambs. We would then splice new jambs, join them into the new sill and refit. The new sill would be packed up to marry the jamb joint and secured.
Hope this helps.
Spike