I called the local distributor and as I expected he doesn't know anything about that.
He's going to ask about and give me prices and delays.
For info: http://www.atb-automation.be/index_F.html (site on french or dutch)
I think that the suitable converter is the following:
http://www.elmomc.com/support/manuals/PL_UG_1102.pdf
Technical Specifications
Pulse and direction input interface:
Feature Details
Input type Optically-isolated
Frequency range 0 - 2 MHz
Input voltage 5 V ≤ Vin ≤ 9 V