Using Powershell can you insert a macro into Access?
Doing this via a macro seems to be an odd and unnecesary step. In theory you should only need to create the new schema once. 

I would create the new schema in postgress and use an ETL tool to map between them. 

I would export the access schema and convert the names to lower case using an editor or as part of the export process. This link gives details on how that can be done with vba https://www.techrepublic.com/article/exporting-access-schema-using-xml/

Another option is to import the schema and then use an alter table statement to rename the columns using lower case. It shouldnt be too hard to write something that can be reused.