Adding another relay to the existing solenoid (relay + mechanical motion) is a mystery to me as well. Are you planning to run another battery cable to the new relay, then to the starter/solenoid? Then this might work, but as you correctly heard from others, it is a bandaid.
If it's a source voltage problem (borderline battery?), then fix that. If it's excessive voltage drop between source voltage and the starter solenoid, fix that. If it's a borderline starter solenoid, fix that. If it's an overheating starter motor ..... (armature expands and sticks or slows rotation speed), take a coffee break and try again later or replace the starter.
Sherwood Lee
http://members.rennlist.org/911pcars