If you plan to marry, you can do that and then sponsor her for permanent residence on an inland PR application. She should include, in the same envelope with the PR application, an
application to change conditions or extend status and that will protect her temporary status in Canada beyond the expiration of her original work visa - as long as the paperwork is received at CPC-Vegreville BEFORE that status expires. It will also result in her being issued a new (open) work permit at the same time that she receives Approval in Principle (which currently takes 4-5 months from when CPC-V receives the file), so she could, conceivably, go back to work for the same employer (or a different one) when she is ready. Finalization of her PR status can take anywhere from 12-18 months.
Your marriage will not necessarily look like a marriage of convenience, but just the fact that she's pregnant will not be enough to convince an Immigration Officer that you are a genuine couple. You will need to demonstrate, through submission of proofs and evidence like photos, cards, gifts receipts, letters from family and friends, etc., that your relationship is not about facilitating her stay in Canada.
Also, as Leon pointed out, an inland applicant is advised not to leave Canada during processing of the application - especially if s/he is the citizen of a non-visa-exempt country - so you might want to re-think the trip to Korea until after PR is approved. This is because, if she is required to apply for a visitor visa to come back to Canada, and she can't get one, her inland application will be lost. In order to be approved as an inland applicant, she must be residing with you (her sponsor) in Canada, and must be able to attend her landing interview in Canada.