perhaps I not really understood a question, but I want to offer a variant who as to me shall seems to approach.
Sense such - In Workflow you create a status - "Set Variables" with automatic transitions for each variable with the necessary conditions and settings.
If it is necessary you send the file from "Initial" to "Biblioteca" through this status.
It is possible to make the separate folder, for example - "Import" for this separate Workflow.
Together with it it is possible to use also Dispatch.
If I am not mistaken for creation of a property in the file it is possible to use "Space",
because if to leave a variable empty - the property in the file will be deleted.
If configurations are necessary - do not forget them to set up.
Automatic transitions can be made parralelny or consistently (then each variable shall have the status.
It is not sure that I clear described everything (something can I forgot). It is possible to eat also other decisions.
But this information can will the useful or will prompt the possible decision.
As other varian - use task - Design Checker.
I wouldn't really want to do it using a automated transition as the files may be in different states within the workflow, therefore it could end up going to a different state in the end.
I am unable to verify this as I'm away from work, but I think you could use Dispatch to check out files, read the values for your Supplier and Supplier Part Number variables and update them (with the same values) and then check them in. I think this will be enough to create a new version of the file with the custom properties.
That has got it to work.
Here is how I have got it set up:
Overwrite latest version to reduce on the number of versions made to component with a minor change made.
The For All Documents/End All Documents loop ensures it takes the variables from each part number, one at a time.