The Operation drop-down in the SourceSafe action is not exhaustive, but it is editable, so you could try entering operations that aren't pre-populated in the list, such as Rename (it may or may not work since they aren't explicitly supported and some operations require certain flag combinations). If that doesn't work for the rename operation, as you mentioned, you could instead use a Run Program step with that uses %SSHELPER%, constructing the command-line manually.
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