Boni, a centre joining from Rugby Mogliano, was called up to Jacques Brunel's Italian squad earlier this year as injury cover.
And De Rossi is confident the squad now has the strength in depth needed to challenge higher up the GUINNESS PRO12 table.
"We are pleased to present three excellent prospects," he told the club website.
"Boni and Canna will further rejuvenate our team bringing with it the potential and quality already shown at other clubs and national youth teams.
"Van Zyl instead is more of a stranger with good experience in competitive championships in South Africa.
"We are happy to continue to unveil for next season a mix of experience, growth and enthusiasm which will be essential to meet the anticipated results on the field, also with a view to better prepare these kids for a possible international career."
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