Ryan has previously played for European heavyweights Munster and French Top 14 semi-finalists Toulon as well as English outfit Newcastle Falcons.
The 27-year-old played against the Dragons twice in last season's Amlin Challenge Cup for Italian side Cavalieri having previously reached the final of the same competition with Toulon in 2010.
Nimmo heads to Rodney Parade from English Championship finalists Cornish Pirates for whom he featured against London Welsh on Wednesday night.
The 26-year-old was named in the RFU's Championship Dream XV this season having previously played for Nottingham, Coventry and Edinburgh and been on the books at Leicester Tigers.
Nimmo represented Scotland at U19 and U21 level and played in three junior World Cups.
The 6'7" forward will replace Dragons captain Luke Charteris after the Wales international agreed to join Perpignan next season.
"I'm looking forward to working with both Tim and Ian," said Dragons director of rugby Rob Beale.
"They bring with them a wealth of experience from playing across Europe and this will only help strengthen our pack going into next season.
"We have been impressed with how they have performed this season and hopefully they will continue in that same vein here at Rodney Parade."