The attached posting gives some perspective on derating three phase Yaskawa drives for single phase input. But as mentioned this applies to other models that may be a bit more heavy duty. The derating does vary by manufacturer and model, I have usually seen something around a 1.6-1.7 derating factor.
If you have an RPC, why not just run the VFD on 3 phase off of your RPC and get the full VFD rating? Otherwise if using single phase, they recommend a DC line choke, and I do not know if your model can be fitted with one, let alone the cost. At the end of the day, it may be more worthwhile to sell your VFD, and buy a decent single phase sensorless vector VFD 3Hp drive (the Yaskawa model you have does not have sensorless vector ability, which can give more low end grunt on a lathe). Probably come out pretty even on the cost in the end.