It's very simple:
First of all you define your custom mechanism and key type as:

int myMechanismType = 0x80000002;
int myKeyType = 0x80000001;

Then create a template:

CryptokiCollection template = new CryptokiCollection();
template.Add(new ObjectAttribute(ObjectAttribute.CKA_KEY_TYPE, myKeyType));

Then create the mechanism:

Mechanism myMechanism = new Mechanism(myMechanismType, null);

and finally call GenereteKey

Key key = session.GenerateKey(myMechanism, template);

and that's all.

In the current version (1.4.5.7) of NCryptoki you should marshall the parameter as byte[] by yourself.
In your case you should prepare a byte[] containing in sequence the values of your struct.

For example if you have:

typedef struct CK_MY_PARAMS {
   CK_ULONG vSrc;
   CK_BYTE vDst [8];
} CK_MY_PARAMS;

with

vSrc = 0x10
vDst = {1,2,3,4,5,6,7,8}

you should create a byte[] as follows:

byte[] param = new byte[] {0x10, 0x00, 0x00, 0x00, 1,2,3,4,5,6,7,8}

Currently I'm working on the new version 1.4.6.x that should make this work for you. but I need some days more to finish development and testing.