It seems to me that a buck boost won't do it. The phase voltages are already 120V from the neutral while they are only 208V apart. It seems to me that if you boost the phase voltages to 240V apart they will be farther than 120V away from the neutral.
depends on whether you use buck boost in a wye configuration or a delta configuration. Most common would be an open delta configuration, in that arrangement one of your supply leads is still common to the output leads and is still 120 volts to the supply neutral. You would most likely use this for boosting voltage to an individual utilization equipment that primarily needs 240 volts three phase, though if it also has limited 120 volt single phase it could be connected to that "common" lead. I'd think it would be best in most cases to still utilize 208/120 source to supply your general 120 volt loads, and whatever loads you can that will work @ 208 (single or three phase) and then put whatever needs to be 240 on buck boost or even separately derived system - depending on what analysis of the situation.
Keep in mind a 208 system that typically runs on high side (like about 215 volts) isn't all that much lower than a 240 volt system that is running near the low end of general accepted tolerance range.
240 less 5% is 228, 208 plus 5% is 218.
That doesn't matter to loads that don't use the neutral, and also applies to deltas with derived neutrals.
There is another thread, fairly current about buck-boost and 208 to 240 volts, with pretty good discussion on wye vs open delta configuration of the buck-boost transformer(s).
