Answer
Yes, through repeated imports rather than a single bulk operation: export each source permission set you want to combine to its own XML file. Import the first one into a brand-new permission set (or start from a renamed copy of one of the originals). For each additional permission set you want folded in, re-import using that exact same source filename each time (so the import process recognizes and matches it), and choose the 'Merge' option instead of a fresh import — repeating this Import-as-Merge step for each additional permission set combines them all into the one target permission set.