Can I use a pen knife for writing as well?

This really depends on what you mean by “pen knife” and by “use for writing.”
An actual pen knife, the type of knife that has the name “pen knife,” is a small pocketknife used for sharpening pens. At the time this knife was invented, people wrote with quills, which are large feathers. The points would get dull so people used a pen knife to sharpen them.
So if you are asking about an actual pen knife, the answer depends on what you mean by “use for writing.”
If you mean writing with the pen knife itself, then the answer is “no.”
If you mean using the pen knife to keep the quill sharp so you can write, then the answer is “yes.”
If you are using the term in the most common usage, as a synonym for any small pocketknife, then the answer is “no,” unless of course you are writing with a quill.
If instead you are asking about a novelty knife disguised as a pen, then it depends on whether it is the type that simply looks like a pen or whether the knife is built into an actual pen. The one I bought in the 1980s simply looked like a pen and could not write. Most models today are built into actual pens and can write. Almost all are not actually practical knives for either utility or defense.

Wanbasion Black Stainless Steel Knife Set, Sharp Kitchen Knife