The 27-year-old former Morecambe, Lincoln City and Macclesfield Town defender joins the Bluebirds on non-contract terms.
Diagne spent his youth career at Nottingham Forest before returning to France.
In 2011 he joined Macclesfield, then of League Two, and made over eighty appearances for the Moss Rose outfit, scoring 6 goals.
He then made the move to Morecambe, after a spell on loan at Lincoln City, where he made over thirty appearances.
In 2014, he signed a two-year deal at Lincoln, before being loaned back to former club Macclesfield.
He spent last season at Hemel Hempstead Town, playing over thirty times in the National League South and scoring twice