You can use a Softpro/Kofax signature pad in thin client Citrix sessions using the Softpro SPVC Signature Channel. The Virtual Serial Sign Pad method is no longer supported as of LX 5.06.100.
On the Thin Client
On the VDI Server (Windows)
The driver contained in this software will listen for signature pads on a virtual channel. Applications such as SignDoc will be able to use the signature pad.