Girolami said: "It's definitely the best opportunity I have had in my career and I cannot wait to start.
"I have a lot to learn about the car, the tracks and the championship so I will be taking things step by step.
"I would love to come to the WTCC next year and race for a full season.
"Racing against 'Pechito' [Lopez] would be the ultimate because he is the reference in the WTCC.
"This is what we are working hard towards achieving but we have to take things slowly and keep the focus on these two races before we think too much about the future."
Although the Civic is not quite a match for the dominant Citroen C-Elysee, independent runner Norbert Michelisz and factory driver Tiago Monteiro have won reverse-grid races in 2015.
Rydell scored points from lowly grid positions in the opening rounds at Termas de Rio Hondo in Argentina, but then missed the following rounds because of a thyroid-related illness.
He will return to the cockpit at Paul Ricard at the end of June.
Team boss Nicklas Karlsson added: "We want to thank Rickard for making this gesture to allow Nestor to drive his car in two races, especially as he's not been able to take part in all the races this season because of illness.
"It's never easy for a driver to do this but Rickard understands the importance of allowing our team to give this option to an upcoming driver and we really do appreciate that.
"Besides, we will take that opportunity to evaluate the feasibility of a two-car team in the future."
Volvo gave the World Touring Car Championship a major boost when it announced it would enter the series, but its ambitions don't stop there. JACK COZENS examines its programme, the S60 and what its arrival means for the WTCC
The 2012 World Touring Car champion says he never had the money to race cars. Yet 2015 is his 11th season in the WTCC. He talks STUART CODLING through his journey from motorsport fan to paid professional - for little more than £100,000