One can roast at home to get an understanding of roasting.
Depending on one's budget, one can start in a pan on the stove or in the oven, to using a popcorn popper, to buying a home roaster, to buying something like the huky 500.
If you work at a company that roasts, just tell someone you are interested in learning more and hopefully there will be a way. Sometimes one just helps with packing and other related tasks and follows a roaster around. Sometimes there is training program to get you going on this path.
I skipped over the barista part of the career progression and went straight to roasting. I was trained as a cook, so I suppose this may of helped me feel comfortable roasting. I read and was active on social networks and was able to connect with a lot people from all over. I roasted at home for 5-6 years and then I started my own roasting business. [I skipped working for someone else too] I offer unique coffees that I get via the connections I have made.
There are many different paths to becoming a roaster, but the biggest part is to just start roasting and sharing your desire to learn with others.