We added 20m to our chain last spring. There are 'c' links made for the job wich are sized to the appropriate size of your chain but - big warning - make sure yours is well made with high quality steel
. On recommendation by the guy who carried out a test on the items for Yachting Monthly we bought ours from a specific engineering firm in the UK. We then had it professionally attached, as we certainly couldn't have hit the rivet pins hard enough. This is not a problem at all as it just goes through the windlass
like the other links.
If you can afford the time/trouble/money or have the skills then having it professionally welded is obviously a very good move. But it does mean getting your whole chain to the welders shop and by the time you've managed all that a new chain might look attractive.
There are plenty of people who never say join your chain. We are comfortable with what we did, but are glad that it is at the 60m mark (we now have 80m of chain, or 262ft) so in fact we don't use it that often.