A diagrma might help to be sure of exactly what you are trying to do
Cheers,

John.

New Extension Existing Building

|------|------|------------|----------------------------
Floor | | | |
Joists | | | |Old Gable Wall
| | | |
| | |------------|
| | |------------|
| | | Header |
| | | Joist |
| | | |
| | | |
| | | |
| | | |
| | | Stair |
| | | Well |
|------|------|------------|----------------------------

(Not to scale.......obviously ;-) )

Hope this makes sense. I've never tried this before.

Regards
Chris