The actress, whose theatre credits include Chicago, Guys and Dolls, Educating Rita, Chitty Chitty Bang Bang and Legally Blonde, will join the production when Flack finishes her contract at the end of the year.
The Gershwin brothers' musical is directed by Paul Hart and features a cast including Tom Chambers and Charlotte Wakefield.
In her review of the original Watermill production, Judi Herman called the show 'a crazy summer cocktail'.
It has music and lyrics by the Gershwin brothers and a book by Ken Ludwig. Songs include "I Got Rhythm" and "Nice Work If You Can Get It". The plot is based on the Gershwin's hit Crazy Girl where a young man is sent to look after his family's ranch but instead employs showgirls from Broadway as entertainment.
Crazy for You is touring to Southend, Nottingham, Bristol, Glasgow, Birmingham, Norwich, Sheffield, Southampton, Manchester, Liverpool, Canterbury, Dartford, Malvern, Wycombe, Truro, Ipswich, Llandudno, Wimbledon, Milton Keynes, Aberdeen, Edinburgh, Bradford, Sunderland, Swansea, Wolverhampton and Woking.