if the arp is synced to midi clock you can set the clock divider / tempo via midi or via the config menu.
If you mean: Set the tempo of the internal arp clock via midi:
It depends. Do you have a Juno-60 or a Juno-6 ?
It is possible for the Juno-60, if you omit one of the Juno-66 CV outs (filter or pitch bend).
If you have a Juno-6, it is however so complicated I wouldn’t bother to try it…
edit: Correction. The arp-clock speed can not be CV-controlled. The LFO speed however can in case of the Juno-60. If anyone wants to know how, ask here and I’ll post an explanation…