Welcome to the community, Coline! :grinning_face_with_big_eyes: If by “populate a multiple select field” you mean that you want to automatically create the list of options for the field—not necessarily which ones are chosen for specific records—then that may be possible depending on the setup for your table. When converting between different field types, Airtable will try as much as it can to maintain the data in the process. For example, if you have a single line text field that looks like this:
…and you change the type to multiple select, it then becomes this:
The same applies if the content in a field is generated by a formula.
As I said above, the ability to do this in your specific case depends on the setup of your table, and whether or not the field data that you want to use to build your multiple select field can all be gathered using links, lookups, formulas, etc. If you can share more specific details about your table layout (screenshots would be awesome!), that would make it much easier for us to offer specific directions.