Connecting a second ground wire from the clamp screw to the outlet should be safe as long as it is done right.
Professionals would have the equipment to test the outlet's ground connection properly.
You'd need to check local code/law to see if it is permissible. This depends on location. In the US you may be able to connect the existing wire to two new short lengths using a wire-nut and then secure the new wires to box and outlet respectively.
See also Can I extend an electrical ground wire with a copper wire crimp?Can I extend an electrical ground wire with a copper wire crimp?