well, its perfectly obvious you can't do that in step record in the composer with a resolution of a thirty second note step available, though triplet timing might get slightly closer. Nevertheless that is not the same as not being able to do strums at all since you can produce acceptable strums by step for patterns at all reasonable tempos if that is what you wish to do.
For something like that I would reduce the tempo and input in real time. Or do the same in the sequencer in Cmajor in R1, tidy in note edit to the required steps, then copy to an unused composer accomp 5, then pattern copy just that accomp5 strum merging into the rest of the composer pattern to which you want the strum added.
Or a few more steps, first play the whole intended composer into the sequencer, apc to smf convert, add the strum to an empty track, note edit to tidy as before and copy the whole pattern back to a composer from the sequencer.