* Adding Maxdome, Eurosport Player won't create dominant player
* ProSieben seeks to challenge U.S. giants Amazon, Netflix
* Adding new partners would draw scrutiny - cartel office (Adds cartel office comment, context)
FRANKFURT, July 23 (Reuters) - Germany's anti-trust regulator said on Monday it had given its blessing for German broadcaster ProSiebenSat.1 Media SE and U.S. Discovery Inc to expand their joint video streaming platform.
The expanded 7TV venture will include content from ProSieben's Maxdome platform and Discovery's Eurosport Player. The duo said they were open to new partners joining when they announced their scaled-up cooperation last month.
The Federal Cartel Office said that the expanded offering - including Maxdome's entertainment content and the airing of German league soccer matches on the Eurosport Player - would not gain a dominant market position.
"The market for paid video on demand continues to expand rapidly and contains potent competitors in the form of Amazon, Netflix, iTunes and Sky, as well as offerings from public broadcasters," its head, Andreas Mundt, said in a statement.
The regulator had already approved the creation of the 7TV joint venture last year. It said it would scrutinise any further acquisition of content or partners.
According to figures from Goldmedia Custom Research, Amazon has a market share of 26.8 percent of paid video on demand in Germany, Netflix 22.4 percent and Maxdome 4.3 percent. Only by joining forces do European broadcasters have a chance of competing with the U.S. platform giants, industry executives and analysts say. (Reporting by Douglas Busvine Editing by Maria Sheahan and Emelia Sithole-Matarise)