Yes, you can complete your survey on behalf of your participants by simply accessing the survey link in the same way a respondent would. If you want to keep track of which surveys were completed online and which offline, you could set up two separate copies of your survey and merge or compare the results after, or you could use a hidden pre-population parameter field to record the origin of the responses.
Bulk-uploading of responses from a spreadsheet is not possible in online surveys.
Online surveys (formerly BOS)
Powerful, flexible online surveys