You can’t convert ObjectId into GUID and vice versa, because they are two different things(different sizes, algoritms).
You can use any type for mongoDb _id including GUID.
For example in official c# driver you should specify attribute [BsonId]:
[BsonId]
public Guid Id {get;set;}
[BsonId]
public int Id {get;set;}
ObjectId:
A BSON ObjectID is a 12-byte value
consisting of a 4-byte timestamp
(seconds since epoch), a 3-byte
machine id, a 2-byte process id, and a
3-byte counter. Note that the
timestamp and counter fields must be
stored big endian unlike the rest of
BSON. This is because they are
compared byte-by-byte and we want to
ensure a mostly increasing order.
GUID:
The value of a GUID is represented as
a 32-character hexadecimal string,
such as
{21EC2020-3AEA-1069-A2DD-08002B30309D},
and is usually stored as a 128-bit
integer