Unfortunately, it doesn’t appear at present that Endnote is capable of batch processing search tasks based on the single PMID field. Searching will need to be done by one-at-a-time using the individual PMID identifiers. (You might submit a request on the Products Suggestion section of the forum for batch processing search tasks.)
If you had another field such as author names (in addition to the PMID field) you could import the fields into Endnote and generate corresponding records which in turn could be “filled-in”/updated using Endnote’s “Find Reference Updates” function. However, this method still calls for working through individual records instead of batch processing them. Moreover, it’s not guaranteed that Find Reference Updates may locate any info so you would need to fallback and search Pubmed using the PMID field.