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Do you mean create a solid island in the middle of a TROR sector? That's called sector punching. In 2D mode, draw a sector in the TROR sector, select all the vertices of this sector with right shift, then go ctrl-e as though you're extending a normal sector, it'll then prompt you which way you want to punch it, up or down. Then you need to point at the sector in the top layer and go ctrl-u to break the connection, leaving you with a solid, editable floating island sector that you can do a lot of neat things with.