User-supplied entropy (a.k.a. a "seed") is added as "additional input" to the generation process. It does not affect the accumulation pools and cannot be used by an attacker to control the output.
Remember it's not how "random" your user-supplied entropy is, but how little an
attacker knows about it. Using the current time is no use. If you can provide
32 bytes* of data of which an attacker knows nothing and cannot later discover,
then you have added 128 bits of security strength.
* The bytes must have been selected randomly from the range 0 to 255.
For more details on the security aspects of the random number generator, see the technical details published on our web site.
CryptoSys API also lets you generate nonces - a term used in security engineering meaning "number used once". Use a nonce where random but not-necessarily-unpredictable numbers are required: e.g. for initialization vectors, SSL cookies and random padding data.