Parashath Noah, Laws of Ziziyoth, Page 14:
5. One should not wear a linen tallith even with linen ziziyoth, in order not to come to affix woolen ziziyoth to the tallith by mistake at some later date. However, if it is impossible to obtain any other tallith, it is permissible to affix ziziyoth to a linen tallith - but only linen ones, and only when no other tallith is available.
This learning should be in memory of Maran, HaRav HaGaon Ovadia Yosef, ztz'l.