Unfortunately the pressure is not additive, but as Richard suggests indirectly you might be able to use the manifold pressure as a priming pump that provides 20 psi of pressure pre-pump increasing the efficiency of the pump and sigificantly increasing it's output.
My plan is to use a self priming, self regulationg water pump to apply pre-pump water feed. Shurflo, Flojet, and a host of others make such pumps for on demand water systems in RVs and Yachts. It pumps until a preset pressure (usually 35-45 psi) and then turns itself off until the pressure drops (opening of a faucet or in my case Aquamist pump activity).
To use manifold pressure in the same way you would have to pressurize you water tank. It would have to be able to maintain 20 psi without leaking or exploding. Most poly propylene (think cloudy milk jug plastic) could withstand this pressure with a rubber gasket seal on the lid.
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