Scandal781: Yes it is possible.

If you are setting up manual routes you need to watch the storage rules carefully.
By default the storage rules in the Windmill for Barley and Wheat should be set to "infinite" as they are a basic resource that the Windmill needs to work.
So if you have a manual route set up to deliver 20 of each, then it will continue delivering that until the storage is full.

If you want the Windmill to only keep 20 max, then you need to set the MAX in the storage rules to 20 (and the MIN to 20 as well) that way the Windmill's own transporter will try to make sure that there are 20 of each in storage at all times. BUT the manually set transporter will not be able to drop off more than 20 of each once the MAX has been reached and will stay idle until there is room again for more.

If you want to have at least 20 in the Windmill storage and sell the rest, then you should set the MIN in the storage rules to 20 and the MAX to infinite. If you want the transporter of the Windmill to sell any Barley/Wheat exceeding 20, then you need to check the box in the storage rules that says "sell", then any excess Wheat/Barley will be sold (please note that the Windmill transporter will sell its own products like flour, grain pap etc. with higher priority than the grain.

Another option would be to set the manual route up to also include a trip to the market and drop off the same amount of Wheat/Barley as in the Windmill. The Windmill storage should be set to 20 MAX for Barley/Wheat. That way, the transporter will offload whatever Barley/Wheat they carry and whatever they cannot offload (all exceeding 20, as set in the storage rules) they will keep in their inventory and sell in in the market as that is their next stop.