Our player rankings are generally derived from real-time "average draft position" (ADP) data, being driven by actual drafts. This data is graciously supplied by MyFantasyLeague.com.
As it takes time (a decent quantity of drafts) for this ADP data to become consistent, during the early part of summer through mid-August our rankings are driven by a formula derived from a compilation of expert consensus.
The ADP settings are a collection of combined PPR and non-PPR draft results from redraft leagues. The period of collection is variable depending on the time of the year. A close representation of the ADP list of player rankings can be seen here, but again will vary based on the period.
At this time, we do not offer any functionality to manually change player rankings, nor change the default rankings to any other service or provider's 3rd party rankings. We are constantly adding new features and enhancements to improve the FanDraft experience, however, we currently do not have a date slated for expanding upon a "custom rankings / other services rankings" feature.