How do I convert the definition of E-521 into a curve definition a la Bouncy Castle?
Bouncy Castle does not support curves of the Edwards form (such as the one E-521 has). See related question: Elliptic Curves of different forms
Bouncy Castle's FpCurve supports curves of the Weierstrass simple form. You can convert Edwards curves to this form by a series of (complex) calculations. SafeCurves shows you the necessary equations. First convert the Edwards curve to Montgomery form, then to Weierstrass form.
Unfortunately doing this will result in absolutely huge numbers, which - on top of the fact that Edwards curves are computed faster than Weierstrass curves - results in poor performance.

Appendix: Curve forms

Weierstrass: $y^2 = x^3 + ax + b$
Montgomery: $By^2 = x^3 + Ax^2 + x$
Edwards: $x^2 + y^2 = 1 + dx^2y^2$