suppose 后可以加if吗？

可以 但不要用will

I suppose if you have the money, you can persuade people to do anything.

我想如果你有了钱,你可以叫别人为你做任何事情。

I am certain that it will remain so and that, having been tested for several decades, it will be able to do its duty.

we believe both of these skills will remain viable but not superior to other options.

不可以，“如果”要用whether！另外可以在that从句中用will。
I suppose whether it will remain so？

可以 但不要用will
具体点是
I suppose (that) if .....

不可以