Doing so is filesystem specific. If you're asking about UFS, then technically the answer is 'growfs'. However that requires that you've already done any work necessary to make sure that the device the filesystem is on (slice, metadevice, or volume), is now larger so that the filesystem can be expanded.
How you might make the space available is a rather complex topic that depends on your storage, how you intend to use the storage, whether the filesystem is a boot-critical filesystem, and if you're using any storage management tools like SDS/SVM or VxVM.
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Darren