I have done this, and it has saved my bacon many times on hot days. I used a 'Mr. Gasket 105 pump' (search eBay with those terms).
To solve various fuel pressure problems, I had replaced the OEM pump 2x and rebuilt it once, but cannot seem to hold good pressure on very hot days. I also have tried many other pumps through the years, including the classic Carter electric pump, and the short-lived Holley motor driven pumps (junk, IMO). Do not use the $29 'vibrator' type pumps - they don't put out enough pressure. I mounted my pump very low in the system - below the battery tray. This makes the pump gravity fed with fuel from the tank. I also have a regulator and small gauge on the output. I run the regulator fairly high - something like 7 psi, which is the max pump output. But, I notice that on hot days, the output can drop to 3 psi. I tried running the electric pump at 3-4 psi, but that wasn't enough.
Beware that anytime you run an elec. pump in front of a mechanical pump, you risk a rupture (someday) in the mech. pump diaphragm and flooding the crankcase with gas. To prevent this, you can entirely remove the mechanical pump and replumb the system.
The Mr. Gasket pump is fairly noisy, and I have it powered through a relay and switch, so I can turn it off for testing or safety reasons. It only runs in the 'ign' position, not accessory.
With the junk summer gasoline we now get, this has been the only pump that has reliably solved my vapor locking issues. Note that my carb fan is working, my carb is freshly rebuilt, my fuel filter is clean, etc. Still, after a 15 minute park on a hot day, I can suffer from hot soak problems and the truck will sputter to life with a big black cloud blowing out the tailpipe. The electric pump did improve cold starting, as you can run the pump for a couple seconds to fill the carb before you start cranking.
YMMV - but this is what works for me.
Steve H. - KE0KNH
Colo Springs, CO
'78 FJ40, '89 FJ62, '98 4Runner