Is it possible create more threads that specified in ThreadGroup in Jmeter?

I need to make a test with 10 threads and 10 iterations with a random pause of 2 to 5 seconds between requests

Just add a Constant Timer at the same level as your request Samplers live and put the __Random() function there. Just multiply the values by 1000 because Constant Timer uses milliseconds.

More information: A Comprehensive Guide to Using JMeter Timers

Im not sure about if this will help, since your looking for adding more thread but based in your question, that you can't make it work with ${__Random(2,5)}. I will suggest try adding Uniform Random Timer instead.
This will have two 2 parameter.

• Random Delay Maximum: Where it will decide the max random pause time
• Constant Delay Offset: This is fixed delay time which will be added in random delay.