Yes, addition becomes procedural knowledge. Below is an extract from the book "Smart Thinking by Art Markman", that clearly explains the process :
When you are learning to do addition, the two procedures you have for
adding compete with each other. One of those procedures requires some
effort. You start with the bigger number and then count up. Adding two
and four means starting with four and then counting to five and six.
The other procedure is effortless. You try to remember the answer. If
you finish counting before you pull up an answer from memory, then the
counting procedure wins. If you are confident you have pulled up the
right answer from memory before you finish counting, then the habit
wins. After you solve the problem (by either method), you store
another memory that 2 + 4 = 6. So, each attempt at an addition problem
provides memories that will make it faster for you to remember the
correct answer in the future.
The difficulty with math is there are lots of similar facts. You are
learning 2 + 4 = 6, but at the same time, you are also encountering
problems like 2 + 7 = 9 and 2 + 5 = 7. Sometimes, when you see 2 + 4,
you will also recall some of those similar problems. When you retrieve
these conflicting answers, you are going to be uncertain about which
answer is correct. So you will finish carrying out your counting
procedure before you have an answer from memory. Once you have a lot
of examples of addition problems in your memory, most of what you pull
out of memory when you see 2 + 4 will be other situations in which you
also saw 2 + 4. At that point, you retrieve information from memory
faster than you can count, and so you have a habit.
Logan, G. D. (1988). Toward an Instance Theory of Automaticity.
Psychological Review 95: 492–527.
Schneider, W., and Shiffrin, R. M. (1977). Controlled and Automatic
Human Information Processing: 1. Detection, Search, and Attention.
Psychological Review 84 (1): 1–66.
Shiffrin, R. M., and Schneider, W. (1977). Controlled and Automatic
Human Information Processing: 2. Perceptual Learning, Automatic
Attending, and a General Theory. Psychological Review 84: 127–190.
The two competing systems mentioned above, are actually system 1 and system 2 that are explained in detail by Daniel Kahneman, in his book Thinking, fast and slow.