Short answer. Yes, but not alone. A person under civil interdiction may execute a marriage settlement only if the guardian appointed by a competent court is made a party to it. Without the guardian joining, the settlement is not valid at all.
What the law says
it shall be indispensable for the guardian appointed by a competent court to be made a party thereto
Family Code, Article 79 — Marriage Settlements by a Person Under Civil Interdiction or Disability. Read the full provision →
Interdiction removes management, not the right to marry
Civil interdiction is an accessory penalty. It does not dissolve a marriage or bar a person from contracting one; what it takes away is the power to manage and dispose of property by acts between the living. A marriage settlement is exactly such an act — it decides who will own and administer what — so the law does not let the interdicted person make it unaided. The provision is protective rather than punitive in design: the couple keep their freedom to agree on a regime, but the agreement has to pass through someone the court has put in charge of that person's property.
The guardian must be court-appointed and must join
Two words carry the weight. The guardian must be one appointed by a competent court, so a relative acting informally, a spouse-to-be, or a person holding a power of attorney signed before the sentence will not answer. And that guardian must be made a party thereto, meaning a signatory to the settlement itself, not the author of a separate letter of conformity. The statute calls this indispensable, which is the strongest word available: it is not a formality that substantial compliance can satisfy, and a settlement that lacks it fails as a settlement.
The same rule for other disabilities
The article is not confined to prisoners. It reaches a person subject to any other disability, which is the clause that matters far more often in practice — someone under guardianship for reason of incapacity, for instance. The test is not the label attached to the condition but whether a court has appointed a guardian over the person or the property. Where one has, that guardian must join the settlement on the same terms. Where no court has ever appointed anyone, the provision has nothing to operate on, and the question becomes the ordinary one of whether the party had capacity to contract.
What to obtain before signing
Get the court's appointment papers, not an assurance that a guardian exists — the order naming the guardian, and confirmation that the appointment is current. Read it for what the guardian is authorised over, since guardianship of the person and of the property are not always the same office. Then have the guardian execute the settlement with the couple, before the wedding, in the same instrument. If no guardian has yet been appointed, that appointment has to come first, and the timing of it against the wedding date is the practical problem to raise with a lawyer early.