The pair will be respectively the creative and planning directors of the group's fledgling agency operation, which has been christened Organic.
Keeble, who formerly ran the creative department at Leo Burnett, and Pritchard, the one-time GGT planning director, will work alongside Tom Rodwell, the ex-Burkitt DDB vice-chairman, who joined Sandom last year as the agency's chairman.
Organic holds ICI's Cuprinol and Polycell accounts and has worked on assignments for RHM and Nestle Purina.
Sandom, whose roots lie in branding and design, wants Organic to complement its other divisions, which encompass strategic consultancy, product development, design and research. Last year, it hired Simon Clinton, who ran McCann-Erickson's pan-European General Motors business, as the managing director to extend its above-the-line expertise.
In joining Organic, Keeble and Pritchard will resume a professional relationship that began at the then Boase Massimi Pollitt. Both will spend the majority of their time with the agency but will be allowed to continue some outside interests, which include consultancy and teaching.
The hope is that Organic will enhance Sandom's so-called Star Chamber offering, under which each division works individually but can also come together.
John Wringe, the Sandom chief executive and the former chief operating officer of WPP's Red Cell network, said: "Unlike the big groups, we're doing more than just talking about harnessing the talent of our individual companies to produce integrated solutions."