Unknown...but the truth may be out there.
There is no character by the name of "Sveta" in any previous X-Files episode, as can be determined by searching through a script database, such as the excellent one maintained at http://www.insidethex.co.uk/. It's possible that she appeared but her name was not mentioned, but I cannot reconcile her with any of the abductees we have seen in the previous 9 seasons.
From what we heard in "My Struggle" (the first episode of the new mini-series), Mulder and Scully themselves do not remember meeting Sveta, but they do not seem disturbed by their inability to remember her. This can probably be squared up with the likelihood that they interviewed hundreds of abductees while the X-Files were active.
So the possibilities are that:
Sveta's back story with regards to meeting Mulder and Scully will not be revealed (and that we are to assume that she was just one of hundreds of interviewed abductees); or
her initial encounter with the agents is significant and will be discussed at some point in the new series; or
they did not actually meet her prior to "My Struggle" and the story was fabricated for some reason.
If and when we learn more, I will update this answer. Given that the final episode of the mini-series will be titled "My Struggle II", there could be new details on the horizon.
UPDATE: There is only a passing reference to Sveta in "My Struggle II" that sheds no further light on her past encounters with Mulder and Scully. As a result, the answer above is unchanged.