How would I knit a heart shape into an exixting pattern
Do you want to knit it with a different color, or with a different texture (like a knit/purl pattern?)
Take a look at the chart for the double-knit potholder - it’s in both the patterns section and the advanced techniques section.
You can just use the pattern for the heart and either do it in two colors (non-double-knit) or in a different stitch - garter stitch or seed stitch on a stockinette background, or reverse stockinette and stockinette would be nice.
Give it a try