I haven't heard of Memory expanders for desktop-class memory, and I haven't delved into server CPUs/architectures too much.
Desktop memory is usually capped by the CPU and/or the motherboard's northbridge, depending on the exact generation (Core i7 and later have the memory controller built into the CPU, for example, while earlier generations have it built into the motherboard's northbridge).
Some small form factor motherboards might not have the maximum slots due to size limitation, but if you browse the high-end ($200+) motherboards on Newegg for a given processor type, you should be able to pretty quickly find the maximum supported memory. Most desktop class CPUs currently available will max out at 2 slots per memory channel. You could in theory have a piece of hardware which joins two sticks of memory which are each half of a slot's maximum, but it would be slow, and not cost effective. The cost effective method would be to buy a motherboard which supports the maximum number of slots for that CPU.