There are couple ways to do it. I have done that many years ago (18 years to be more precise) and in that time I didn't have so many tools as we have today.
But here are some hints:
1) Look at IBM/DB2 website for some migration tools available ( I know from oracle or Sybase to DB2 but both are relational)
2) The biggest challenge is the data model. In ADABAS (I worked with ADABAS as DBA and NAtural for more than 12 years) most of data structures (files) have periodic fields or groups which in relational world it is hard to implement. Also ADABAS phonetic descriptors are hard to convert. For those files that have ISN as the file key...it can be converted in DB2 as identity or sequence.
3) Natural ...the easiest way it either have an SQL layer for Natural to talk to DB2 via SQL so that we wouldn't need to convert all your programs or find a tool that converts Natural programs in SQL which could be tricky sometimes.
4) For data extraction and load into DB2 there many tools out there. One of them that I used recently was Informatica PowerExchange which can pull data from anything and load into DB2,Oracle etc.
I hope I could be helpful. If you need anything else we could break down in more details.