I am running a Bitcoin Full Node on Raspberry pi 3b+. Blockchain sync is dead slow because of less CPU. Is there a way to add additional CPU to a pi board. or connecting additional pi to the same Hard Disk to increase the speed?
Adding a CPU is not the way I'd go about it. But if you want to make the RPi a part of your solution, there's a fair amount of work going on in that area now (as @Fabian's comment suggests). A quick Google search turned up this story on OctaPi. It's spun by the RPi Foundation which might give it some credibility, and they claim it's scalable up to 8 RPi - so, you can tailor the compute power to your problem.
I've never heard of trying those strategies. I've never heard of a Raspberry Pi hat or other add-on that adds a new CPU. I suppose you could use the Raspberry Pi Compute Module with multiple modules, but that would be more expensive than choosing a more powerful single-board computer.
There have been people who have made parallel computers with a network of Raspberry Pi's. These clusters are more educational than useful, but they might increase your CPU speed enough, and they should be able to share a hard drive. I don't know if this will help enough, though, because I've never tried this.
Have you considered a more powerful board such as the Udoo x86 series? This is an x86-based board (or rather several of them with increasing speeds and prices). It will run Linux just fine, it can also run Windows. It includes an on-board Arduino clone, which is useful for robotics.
The Raspberry Pi, while a wonderful computer for some things, has too many networking bottlenecks to be useful where networking is important. Don't get me wrong, I like the Raspberry Pi. I have many of them. I'm going through my electronics, and I have many more than I thought I did. Maybe I will try making a parallel Pi someday, purely for educational purposes.