Of course the best thing would be to talk them out of this course of action. You didn't mention how many devices are involved but there are best-practice limits, usually between 256-512 devices in a broadcast domain (VLAN in this case). If it helps over the years the industry trend has been toward smaller subnets, although the limit really depends on the kind of traffic.
I'm not sure that you can assign multiple IP addresses to the same VLAN (Cisco would call them 'secondary addresses’). You might be able to create multiple VLANS with their own addresses and then plug their untagged ports together, but it could be a minefield of issues with spanning tree and such.
In IP terms supernetting typically means referring to several contiguous subnets as one large block. It really only applies to routing tables though since each subnet still has its own broadcast address and devices in those subnets need to have a gateway address that is in their subnet range. For example, if we have two subnets 192.168.2.0/255.255.255.0 and 192.168.3.0/255.255.255.0 in our routing tables we could create an entry for 192.168.2.0/255.255.254.0 that covered the range of both subnets. The final gateways still need addresses in both subnets though.
If the subnets are contiguous you might be able to combine them into a single larger subnet. You’d need to change the netmask and default gateway of every device, probably all at once, and then the default gateway would get a single address in the ‘new’ subnet. One nice part of this approach would be that the device’s individual IP addresses wouldn’t need to change… that’s usually the part that causes problems if the addresses are referenced anyplace.
If they're trying to solve some kind of technical problem perhaps we can offer another solution - if its a political or management issue your probably on your own.
