Isolated power supply for a cascaded multilevel converter
(1) If you want to suppy the cells via DC/DC converters, each one has to provide full insulation according to the DC link of the cascaded multilevel converter. Therefore, you have to get power supplies with the right insulation level. 

(2) Alternatively, you could energize the DC link caps of the cells with a dedicated control scheme of the cascaded multilevel converter. This is not too difficult, see e.g. 

Wim van der Merwe, Peter Hokayem, Lidia Stepanova, "Analysis of the 2-Cell Single Phase MMC Natural Balancing Mechanism", Emerging and Selected Topics in Power Electronics IEEE Journal of, vol. 2, no. 4, pp. 1149-1158, 2014.

(3) In industrial medium-voltage cascaded multilevel converter systems, they employ (1) with dedicated power supplies (emphasis on the high-frequency insulation transformers) or (2).