The BMD-300 and BMD-350 antennas cannot be modified. The BMD-301, however, is specifically designed for use with an external antenna and has a U.FL type connector. The BMD-301 shares the same physical footprint and pin-out as the BMD-300. See section 15.8 of the datasheet for antennas that are pre-certified for use with the module. There are two patch-type dipoles and three whips on the list. Manufacturer part numbers are also noted.
For the USA, the FCC allows substitution of a similar type of antenna (e.g. dipole) with equal or lesser gain without a need to re-certify. Details can be found in this FCC page: FCC Permissive Change Policy
It's best to contact an accredited RF test facility for the best advice regarding any antenna substitutions for the target sales regions for your product.
BMD-301 reports and certifications: