I’m assuming that what you mean is that you have time and other resources to do research, but don’t have direct access to the users in order to conduct observations, interviews, or surveys. There are a number of methods of user data collection that depend on archival sources rather than direct access:
For users of a corporate or business product, you can ask your client if you could see:
Job descriptions and organization charts to understand the responsibilities (i.e., goals) of the users.
Personnel records or reports for extracting the experience and knowledge of the users.
Procedure manuals and policy statements to understand the task and information flows (at least how it’s “supposed to be done”).
Inventory of tools, software, and sites currently used (or at least given) to the users, along with access to the same (or at least to their user manuals) to see what users are used to (e.g., information and menu organizations; symbols, codes, units, formats, terms, abbreviations, and layouts).
For users of a consumer product, you can ask your client for:
Business plans and marketing strategies relevant to the product, to identify the ultimate goal or position of the product.
Market research results, to identify characteristics about the user and the specific needs the product is aiming to fulfill.
List of existing competitor products for study, to identify what users are used to (or at least inclined towards), and also to determine what can be improved in the UX (by cognitive walkthrough or heuristic analysis).
For either, ask for:
Hit logs and database entries and reports for the current system to see what users are currently doing (e.g., how often they engage in each activity, what sort of entry errors they make).
Records or reports from tech support, complaint departments, customer service, and critical incidents for identifying problems with the current system.
Of course, nothing beats being face-to-face with users to see what really matters to them. But the above methods are good as a substitute or at least a supplement to limited direct user access.
I’d be careful about making a persona too detailed or refined if you have limited or unreliable user data. Rich personas can be very compelling, which would give your product team members an illusion of greater accuracy than there really is. In this case, you may want to choose a different way to summarize your research findings that highlights the uncertainties to your team.