Can I replace Pt100 with an Pt1000?
A PT100 has 100 Ω resistance at 0°C. A PT1000 has 1000 Ω at 0°C. Any method you might employ that tries to turn a 1000 Ω device into a 100 Ω device will result in total inaccuracy of measurement at non-0°C temperatures.

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If you put a 111.11 Ω resistor in parallel with the PT1000 you'd get 100 Ω at 0°C but, At 100°C a proper PT100 has 138.4 Ω whereas 1384 Ω (PT1000) in parallel with 111.11 Ω does not produce 138.4 Ω.