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
Connect the signature pad to one of the thin client's USB ports.
Go to Sessions > Citrix Xen Desktop / XenApp > HDX / ICA Global > Mapping > Device Support.
Enable Softpro SPVC Signature Channel.
On the VDI Server (Windows)
Install the required software from Kofax.
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.