Yes, just like a normal scanned 4x3 matrix, you just need to branch out each individual keypress to read from an array to get multi-tap. It has nothing to do with interrupts; you could or couldn't use them depending on your needs.

for example.
your lookup table for a 4x3 routine might look like this

#define STAR 10
#define HASH 11
const char Keytable[12] = {0,1,2,3,4,5,6,7,8,9,STAR,HASH};

 

To now make the same keyboard do alphabets is just an extension of this idea

char    Index;
const char
* const	Keytable[] = {"0 ", "1.", "2ABC", "3DEF", "4GHI", "5JKL", "6MNO","7PQRS", "8TUV", "9WXYZ"};

 

Now, on decoding the matrix, you just need to read the pointed array at its index like this

    Key = Keytable[scankey][Index];
    if (Index < strlen(KeyTable[scankey]))  Index++;
    else   Index = 0;

 

The second and third line are used to move to the next letter in the array and depends on the size of the array.

CAUTION : This code is untested and may not be syntactically right. I hope I've been able to convey the concept clearly.