I would not do it for long term use. 208V is at the low end of the tolerance range for most motors. At this low voltage you can expect lower starting torque and increased running current, also the life of the motor will be shortened.
This is the perfect application for a buck-boost transformer. Your application will require two single-phase transformers connected in an open-delta configuration. Your 20HP motor will need at least 21.5KVA.
For boosting 208V to 230V (10% change) you will need 1.5KVA transformers.
For 208V to 236V (13.3%) use 2KVA.
You can get an on-line buck-boost calculator at http://www.squared.com.
Then go to the Support and Resources section and choose Free Software and On-line tools.