The convert( expr, 'ModifiedMeijerG', x ) command converts a functions in  into its equivalent ModifiedMeijerG representation, whenever possible.  Most standard special and elementary functions can be so converted, along with many complex expressions involving these.

Prudnikov, A.P.; Brychkov, Yu; and Marichev, O. Integrals and Series. Gordon and Breach Science, 1990. Vol. 3: More Special Functions.

Roach, K. "Meijer G Function Representations." In Proceedings of ISSAC '97, pp. 205-211. Edited by Wolfgang Kuchlin. New York: ACM Press, 1997.