Use NIMH mode on your charger and set small charge current, eg. 0.5A to charge the pack up to above 6V (or above the 2 x minimum per cell voltage you have set for LIPO on your charger, eg. 3.0V per cell = 6V), then change back to LIPO mode and balance charge it.
What is the highest voltage you can attain?
If above 2.8V per cell, I think you can set the minimum per cell voltage for LIPO and balance charge accordingly.
Did you set the charger to NIMH mode for 7.2V (6 cells) at 0.5A to charge? How long did you charge it and still cannot charge above 4.8V? Be reminded to charge in a LIP safe bag for safety. It is highly probable that your LIPO can be certified dead.
Yes, I have set my charger to Nimh mode 7.2v and charge It at 0.5A. When it hit 4.8v, the charger said It is full and stop charging. Then I charge It with Lipo setting, the charger said output error.