This is likely not possible to do and attempts to do so will be counter-productive. Trying to force the system to play favorites makes it less efficient. The other programs will still need at least the same amount of CPU as they need now, so the same amount will be left over for the preferred programs. You might improve latency for the preferred program a little bit, but at the cost of more cache refills from the other programs slowing it down while it's running.
Basically, the people who designed your OS and its scheduler knew what they were doing. Unless you know something significant that they don't about your specific use, you're much more likely to make things worse than better.
When you see a program is only using 50% or 25% of the CPU or an even number like that, it's almost always because it's fully loading as many cores as it is capable of using. Getting the program to use more cores generally requires re-designing it.
Some specific programs may be tunable for multi-core. If the program is one of these, and that switch is off, turning it on can definitely help. But most such programs automatically detect multi-core machines. So there's no room for improvement there usually.
You can also cause performance catastrophes this way. Just ask the Mars rover team. For example, imagine if the process you've boosted has two threads and the system has one core. The first thread is blocked on a system resource currently held by another process. The second thread can make lots of forward progress. The first thread can be stalled for a very long time because the second thread can starve the other process such that it can't release the system resource.
This can happen even if the developer of the program specifically lowered the priority of the second thread to try to give CPU preference to the first thread because it was doing important work. The static process priority controls Windows gives you are just too coarse. This will, for example, happen on many file accesses if you have an active anti-virus scanner.