Thank you for your post. Here is a document for the users guide for the SI4093, in it there is a discussion about setting up a trunk. http://publib.boulder.ibm.com/infocenter/flexsys/information/topic/com.lenovo.acc.si4093pt.doc/00cg189.pdf
From the document:
"Trunk groups are also useful for connecting a SI4093 to IBM switches and
third-party devices that support link aggregation, such as Cisco routers and
switches with EtherChannel technology (not ISL trunking technology) and Sun's
Quad Fast Ethernet Adapter."
Since this switches are essentially passthrough switches where you create a SPAR (which is essentially a logical configuration combined INT or internal ports to EXT or external ports) there does not appear to be a way to "stack" these switches since they have little to no intelligence built in. If you would like to stack the switches that would require the EN4093 switch which has an almost identical command set and functionality set to Cisco when you configure it to use the ISCLI (Industry Standard Command Line Interface).
Keep in mind also that the SI4093 can be tricky to configure although the marketing material would tell you otherwise. I have seen this switch deployed at only one customer, the rest have had the EN4093. I do not want to deter you from your solution but simply warn you that it may not be as easy as it seems. There are a few posts on issues that have been seen here in the MyPureSupport community so if you do hit a snag do a search and see if you find some answers, otherwise post again.