No.

So, what is the use of the ARP user memory in montage ?

There are 256 User Arp locations in the Montage. It can load arpeggio data created for the Motif XF (.X3G).

"In terms of sounds, the MONTAGE is fully compatible with the Yamaha MOTIF XF synthesizer. In fact, the MONTAGE comes with all of the MOTIF XF’s 1,353 voices, 3,977 waveforms, and 7,881 arpeggio patterns. Furthermore, voices saved in the MOTIF XF can be imported directly into the MONTAGE. Likewise, libraries for the MOTIF XF, including third-party patch libraries, can be loaded into the MONTAGE and played."

The compatibility between Motif XF and Montage exists for Voices, Waveforms, and Arpeggios.
This includes any user arpeggio data made for the XF.

It is possible to use a motif rack xs to compose ARP and then load in the user ARP montage bank?

The Motif-Rack XS has no ability to create user arpeggio data.

Thank you.