While we don't have a full spectrum color wheel at the moment (we'd like to add one), you can add non-Copic colors to your palettes in a couple ways.
1. Tap+hold on the Copic color dot at the center of the tool wheel to bring up the Colors menu. (Or open the color wheel and tap on the Star icon.) Fill in the HEX, RGB or HSB color codes to choose a custom color, or use the mini spectrum sliders beneath the fields to find a value (easiest with a stylus). The color dot in the upper right corner of the menu will change color to match. Tap+hold+drag on the color dot to move it into a custom color palette. Tap the palette to activate it onto your on-canvas color slider, and access it on the canvas at any time.
2. Import a color reference or photo (such as a full spectrum palette) onto the canvas, and use the color picker to pick the color. To apply it to your dynamic palettes, draw a sample on the canvas with your brush, then open the Colors menu (as above). Tap+hold+drag on the color dot to drag it into a custom color palette, or from your Most Recent Colors dynamic palette, tap+hold+drag the color into a new palette. When you activate the new palette, it will appear on canvas in the color slider - just swipe on the slider to access your palettes.