Realistically possible? No.
Technically possible? I'm not going to say it'd be absolutely 100% impossible, but unless you had the time, skills, equipment (and money) to burn on a side project, it really wouldn't be a practical exercise. By this I mean I can't see a way for you to then be able to use it in the usual manner one uses a laptop (too much by way of modifications required to get it to work in a typical everyday manner).
And most importantly - to address the 2nd question you snuck in there :) - It wouldn't be worth it. I'm assuming you're talking about the 13" models as follows:
- The top 13" 2012 MacBook Pro has a 2.9GHz (Core i7) processor with two cores.
- The bottom 13" 2018 MacBook Pro has a 2.3GHz (Core i5) processor with four cores.
At the risk of oversimplifying things with this analogy, having four cores compared to two cores is like having four people v two people trying to do some heavy lifting. In most cases, four people each capable of lifting 50kg is better than having two people each capable of lifting 60kg.
To put this into perspective, testing both these MacBook Pros using the same benchmarking tests, we get the following results:
- 13" 2012 MacBook Pro (2.9GHz Core i7) achieves a single core score of 3,192 and a multi-core score of 6,194
- 13" 2018 MacBook Pro (2.3GHz Core i5) achieves a single core score of 4,496 and a multi-core score of 16,445
Information on how to interpret these results is available here
So, as you can see, the newer MacBook Pro specs are significantly better than the older model, despite it having the i5 processor at a lower clock speed than the i7 processor.