Richard Beale's youngsters head to the Valencian coastal town of Oliva on 8 July for a week of intense fitness work.
And now added to the end of their seven-day schedule is a match against the Cobblers.
The game will be played on 14 July, kick-off 6.30pm.
Justin Edinburgh's side, who finished 16th in League One last season, are staying in the same resort.
It will be a good test for Beale's boys, facing up to senior players from a third tier club.
Blues Under-23s already have away friendlies confirmed against Stourbridge (18 July) and Leamington (22 July) in the week after they return to the UK.