Asim confirmed in his interview that he has not been approached for this show and this show was only for Sana and Paras from the beginning.
So don't think Sana was forced by the channel. She signed up for the show because it gave her an opportunity and exposure.It is another matter that she is not able to pull it off.
If the channel had confidence in her abilities to pull off a role in one of their fiction shows they would have definitely offered her a role.
Channel wanted to cash in her popularity and they gave her a show where they thought she could do well.
Don't think any force is involved here.
All BB contestants are usually signed in for a year and it is not for any specific show but for appearances in functions promoting Colors channel and performing in channel functions so and so forth.
Shukla and Rashmi are there to provide the needed support for the show MSK because they are channel's big stars.