Multiselect some of the record

Remember that the records and related actions are contextual and there is nothing at the records level as an action, it is at individual record level.
So what csteward has suggested is a good way to keeping the context by having the default as record from where the related action was triggered and allowing to select other records while you enter a process.

An addition to mschmitt has proposed would be to have an action with start form where the start form is nothing but the grid view of all the records (like on the records tab) and from there you select and proceed.
Why a start form? Because it won't create a process till you actually have selected records and started editing, thus no side affect till you have made the selection like you do on the records tab

Remember that the records and related actions are contextual and there is nothing at the records level as an action, it is at individual record level.
So what csteward has suggested is a good way to keeping the context by having the default as record from where the related action was triggered and allowing to select other records while you enter a process.

An addition to mschmitt has proposed would be to have an action with start form where the start form is nothing but the grid view of all the records (like on the records tab) and from there you select and proceed.
Why a start form? Because it won't create a process till you actually have selected records and started editing, thus no side affect till you have made the selection like you do on the records tab