Custom routes would be nice, but for a system whose basic mathematical routing is still largely a work in progress because of limited map resources that still need a lot of input, the custom route support just seems a bit out of scope for now.
If you really think a certain way is better than what Waze suggests, go that way anyway with Waze turned on. This will enable you to edit the map along that route. Make sure the road segements along the route in question are of the correct type according to the guidelines in the wikis. This has an impact when the distance between your origin and destination is more than a couple miles and the impact gets bigger as the distance gets larger when certain smaller road types start to get ignored. Also, check that all the turns you made are allowed. Remember that by default, Waze assumes that any possible turn is not allowed until it actually sees a track that makes that turn. Waze might discard an entire route or large section of route from consideration if it thinks a certain key turn cannot be made.
Turns out, in my experience, I've found that most routes (after being fixed up in the map editor) I thought were faster than what Waze came up with were actually not faster, Or the actual difference in time was so trivial that the routing server could tip a tie-breaker either way based on reports of even the slightest traffic conditions.
A computer doesn't necessarily do what you want it to do; rather it does what you tell it to do.
Wazing the Garden State... one jughandle at a time.
AM: New Jersey, LEVEL 4
AT&T iPhone 5 / iOS 7.0.4 / Waze 220.127.116.11