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.
Don't know much about nodes, but there are some cluster PC projects using USB or WIFI for the Zero. I'm sure they can also be applied to other Pis like the 3B and 3B+, or 4B.
You can always overclock by +100MHz from stock (ram, arm, GPU, core) , for slightly higher performance.
Also, see if you can install an SSD instead, to lower latency, and disable bluetooth, if you want your network not to throttle (big issue on Pi3B+)