There are two types of infrared filter; infrared cut-off filters and infrared passing filters. Infrared cut-off filters prevent light in the near infrared range of the electromagnetic spectrum from reaching the sensor. Infrared passing filters allow this light to pass, but block light in the visible range of the electromagnetic spectrum from reaching the sensor.
For infrared photography, the type of filter you need is the ‘infrared passing filter’ so that near infrared light can reach the sensor, but most visible light is blocked.
If you had your camera converted to infrared you may still use external infrared filters. Just keep in mind that the external filter needs to be stronger or deeper in the IR than the internal filter. For example, if your camera was converted to the Super Color Infrared filter, which is 590nm – you can use a 665nm or 720nm or even deeper to override the internal filter. Because what you are doing is blocking a wider amount of IR than the internal filter. This wouldn’t work the other way around, say if you had a Deep BW infrared filter (830nm) installed in your camera and wanted to use a 590nm filter – although the external 590nm filter blocks light at 590 and less nm, the internal blocks more, effectively blocking whatever the 590nm filter passes.