The livewell is a raw water device. In order to add a true fresh water washdown, you need a fresh water tank, fill and vent fittings, a wash down pump, a screen/filter, associated plumbing and a deck or gunnel wash down hose fitting.
If you boat in 'fresh' water than you could add a washdown pump and plumbing. I would still call that a raw water washdown.
Some folks do use a livewell pump output through a selector valve to create a raw water washdown source. It will work but as you suggested, it make provide a weak flow.
Better approach is to get a dual port livewell pump ( you may already have one), connect the secondary output to a new washdown pump and all the previously mentioned parts.