We report the finding of kiloparsec (kpc)-scale radio structures in three radio-loud narrow-line Seyfert 1 (NLS1) galaxies from the Faint Images of the Radio Sky at Twenty-centimeters of the Very Large Array, which increases the number of known radio-loud NLS1s with kpc-scale structures to six, including two \ensuremathγ-ray-emitting NLS1s (PMN J0948+0022 and 1H 0323+342) detected by the Fermi Gamma-ray Space Telescope. The detection rate of extended radio emissions in NLS1s is lower than that in broad-line active galactic nuclei (AGNs) with a statistical significance. We found both core-dominated (blazar- like) and lobe-dominated (radio-galaxy-like) radio structures in these six NLS1s, which can be understood in the framework of the unified scheme of radio-loud AGNs that considers radio galaxies as non-beamed parent populations of blazars. Five of the six NLS1s have (1) extended radio luminosities suggesting jet kinetic powers of >åisebox-0.5ex~ 10$^44$ erg s$^-1$, which is sufficient to make jets escape from hosts' dense environments; (2) black holes of >\i̊sebox-0.5ex~ 10$^7$ M $_☉$, which can generate the necessary jet powers from near-Eddington mass accretion; and (3) two-sided radio structures at kpc scales, requiring expansion rates of \rs̊ebox-0.5ex~0.01c-0.3c and kinematic ages of >\rae̊box-0.5ex~ 10$^7$ years. On the other hand, most typical NLS1s would be driven by black holes of <\raib̊ox-0.5ex~ 10$^7$ M $_☉$ in a limited lifetime of \raiso̊x-0.5ex~10$^7$ years. Hence, the kpc-scale radio structures may originate in a small window of opportunity during the final stage of the NLS1 phase just before growing into broad-line AGNs.