Well, it depends on particular situations.

If critically endangered refers to a particular population, but there is additional germplasm available in other areas, you could just proceed with some introductions. Whether private alleles will be kept or not is serendipity.

If the species is critically endangered in the whole distribution area, you could translocate plants/progenies among locations, and hope that future crosses among the most distant phenotypes produce some type of "hybrid vigor"..

indeed, you could mutagenize the gametes during meiosis, but this would produce a very large genetic load.

Theoretically, it is possible that during course of time IUCN status of a species changes from endangered to lower risk.

Good questions.

I will respond in two points:

1) To increase genetic diversity of the endangered species, in addition to making introductions (see Pablo's post above), you can molecularly characterize your population using for instance SNP markers, study available polymorphism, check the existence of genetic structures (subpopulations), and calculate genetic distances between individuals in the population, on the one hand, and calculate genetic distance and/or the amount of allele frequency divergence between the identified subpopulations (there are several free algorithms to help you out on this one). Now, you can proceed with planning controlled hybridizations between most genetically distant individuals. Crossing most genetically distant individuals will most certainly boost genetic diversity. most genetically distant individuals can be individually identified thanks to computed genetic distances (e.g. Euclidean distance), or they can be fetched from most divergent subpopulations (identified using allele frequency divergence).

2) To rescue endangered species, you most urgently have to organize collection expeditions to collect seeds (should the species propagate with seeds) for ex-situ conservation. Contemporaneously, you can protect the species just where it is (adapted): this is in situ conservation. Here you will do good for this species should you be able to lobby politicians/policy makers to issue a law/decree enforcing the protection of the concerned species.   

Hope this helps.

Good question and you got good answers, but can I respond with a question: why do you want to increase diversity. In the first place do you know why the genetic base was narrowing? was it due to natural processes or man-made changes in the environment?  If the latter there a good chance that measures proposed by our colleagues as above will be fruitful, but if due to natural processes it will be futile to try to reverse it. I think you need to be very cautiuos in introducing any germplasm into the region.Your best bet is as pointed by Ephram to loby for more stringent protect and reverse the conditions that original caused depletion of diversity.

• By following the IUCN guidelines mixing with local ethical values of that area.
• By creating awareness and capacity building of the target population groups.