Mission cargo is specified by a string, and as you know cannot be bought or sold in the commodity exchange. A jünk resource is bought and sold in the commodity exchange, and cannot be used for a mission's cargo.
However, you can easily make a mission whose cargo has the same name as a jünk resource. In Nova Data 5 look at STR# resources 4000, 4001, and 4002. Strings numbered 1-80 are used by the default scenario (however, number 80 only appears in resource 4000) so for a plug-in you will want to make your cargo strings start at number 81.
To do this, copy STR# resources 4000, 4001, and 4002 from Nova Data 5 into your plug-in. Open one of them, create string 81, and enter the name of your cargo. Repeat for the other two. If you are matching the name of jünk from the stock game, you can find those strings in STR# 4005 of Nova Data 5, so copy the text you want and paste it where you want. I believe you must copy one entry at a time.
Then in your mďsn resource, enter the cargo type corresponding to the string number you used, probably 81 for your first one, and increasing from there. Note there is an engine limit of 256 cargo types.