Suppose I'm designing a software application that requires high bandwidth / low latency memory transfers to operate properly. My OS uses Virtual Memory addressing.
Is there a way to enforce the variables (that I choose) to be located in DDR and not on the hard drive for example?
You're conflating virtual memory with swap memory: Virtual memory just means, that the address space in which a process operates is an abstraction that presents a very orderly structure, while the actual physical address space is occupied in almost a chaotic manner. And yes, virtual memory is part of memory page swapping, but it's not a synonym for it.
One way to achieve what you want is to simply turn off page swapping for the whole system. It can also be done for specific parts of virtual address space. But before I explain you how to that, I need to tell you this:
You're approaching this from the wrong angle. The system main memory banks you're referring to as DDR (which is just a particular transfer clocking mode, BTW) are just one level in a whole hierarchy of memory. But actually even system main memory is slow compared to the computational throughput of processors. And this has been so since the dawn of computing. This is why computers have cache memory; small amounts of fast memory. And on modern architectures these caches also form the interface between caching hierarchy layers.
If you perform a memory operation on a modern CPU, this memory operation will hit the cache. If it's a read and the cache is hot, the cache will deliver, otherwise it escalates the operation to the next layer. Writes will affect only the caches on the short term and only propagate to main memory through cache eviction or explicit memory barriers.
Normally you don't want to interfere with the decisions an OS takes regarding virtual memory management; you'll hardly able to outsmart it. If you have a bunch of data sitting in memory which you access at a high frequency, then the memory management will see that and don't even consider paging out that part of memory. I think I'll have to write that out again, in clear words: On every modern OS, regions of memory that are in active and repeated use will not be paged out. If swapping happens, then, because the system is running out of memory and tries to juggle stuff around. This is called Thrashing and locking pages into memory will not help against it; all it will do is forcing the OS to go around and kill processes that hog memory (likely your process) to get some breathing space.
Anyway, if you really feel you want to lock pages into memory, have a look at that mlock(2) syscall.