Can I use pen ink for writing brush?

No way! ! Since you use a brush, that's the stroke you want to write. Pen ink is diluted, and its composition is very different from that of ink. Ink can stick strokes together so that you can draw a stroke. If you use pen ink, you can write one or two crosses. This is definitely impossible. If you think pen ink is convenient, I suggest you buy a soft pen, which looks good to write. Similar to brush strokes, you use pen ink.