Short answer. Article 623 of the Civil Code offers two cures. Where an easement cannot be acquired by prescription and no document shows its origin, the gap may be filled either by a deed of recognition executed by the owner of the servient estate, or by a final judgment of a court declaring that the easement exists.
What the law says
The absence of a document or proof showing the origin of an easement which cannot be acquired by prescription may be cured by a deed of recognition by the owner of the servient estate or by a final judgment.
Civil Code, Article 623 — Curing the Absence of a Title. Read the full provision →
Why long use alone is sometimes not enough
Not every easement can be acquired simply by using it for long enough. The Code allows prescription only for easements that are both continuous and apparent; those that are discontinuous, such as a right of way exercised only when someone actually passes, or non-apparent, showing no visible external sign, cannot be acquired that way no matter how many decades the use has run. For those, use proves habit, not right. Article 623 exists for exactly that situation: the burden has been accepted in practice for years, and there is simply no paper explaining where it came from.
The first cure: a deed of recognition
The cheapest route is agreement. The owner of the servient estate — the property that carries the burden — executes a deed acknowledging that the easement exists, and that document takes the place of the missing title.
Have it notarised, describe the route or the burden precisely, identify both properties by their certificates of title, and register it with the registry of deeds so it is annotated on the titles. An unregistered deed can bind the person who signed it while leaving you exposed to a future buyer of the same land who purchases without notice, which is the very problem you were trying to solve.
The second cure: a final judgment
If the neighbour will not sign, the remedy is judicial: an action to have the easement declared and defined, ending in a final judgment that does the same work as the missing document. That is slower and costlier, so it is worth attempting the deed first, and worth documenting the refusal.
Either way, collect your evidence now rather than later — old survey plans, tax declarations, photographs of the path or the drainage line, sworn statements from long-time residents, and any correspondence in which the neighbour acknowledged the use. Where the easement is a right of way and no easement can be established at all, a separate remedy exists for an owner whose land has no adequate outlet to a public road, on payment of the proper indemnity.